Prepare the oven and baking dish: Preheat the oven to 350°F. Grease a 2- to 3-quart baking dish with cooking spray.
Combine casserole ingredients: Stir together the first 5 ingredients and ½ cup of the butter until well blended. Spoon into a greased 2- to 3-quart baking dish.
Add casserole topping: Crush the cereal, and stir in the remaining ¼ cup butter. Sprinkle over the potato mixture.
Bake casserole: Bake at 350°F for 50 minutes.
